Physically-driven dissolved oxygen dynamics in the Bay of Biscay and the English Channel

Abstract. The shelf of the Bay of Biscay and the English Channel form a contrasting hydrodynamic environment, with several areas recognised as exhibiting signs of hypoxia. However, research on low oxygen levels in the region has, until now, focused predominantly on the estuaries and small bays. Here we use a three-dimensional circulation model coupled with a simple oxygen model to examine the physical controls on dissolved oxygen concentration at the shelf scale. The model assumes an oxygen consumption rate in the water column that depends on the temperature and dissolved oxygen concentration. Despite its simplicity, the model reproduces a large part of the dissolved oxygen observed variability, thus highlighting the predominance of physical processes in driving dissolved oxygen at the regional scale. The areas exhibiting low oxygen concentrations are predominantly observed in coastal zones, especially within major Regions of Freshwater Influence. The variation in dissolution rates caused by rising temperature is found to be the fundamental factor driving the trend in bottom oxygen levels. Model results demonstrate that bottom temperature also acts as the primary driver of most interannual oxygen variability in Regions of Freshwater Influence, while stratification appears to play a minor role.